1. Field of the Invention
The present invention relates to a sulfur vulcanizable rubber composition which does not contain aromatic process oils. The rubber composition can contain at least a diene elastomer, at least finely dispersed, precipitated silica and carbon black as fillers, softeners, at least one silane coupling agent, and other common additives. Additionally, the present invention relates to a process for producing the rubber composition, and to rubber products and tires made on the basis of this rubber composition.
2. Discussion of Background Information
It is known to add softeners to natural and synthetic rubbers and their rubber compositions in order to lower their viscosity and to improve their processability. For this purpose, preferably commercial products in the form of aromatic petroleum raffinates (aromatic process oils) can be used. Such aromatic oils allow, on the one hand, the improvement of the processability of the highly viscous, uncured components of the compound, due to their effect of lowering viscosity and/or hardness, and, on the other hand, they reduce the hardness, the stress value in the stress test (module) and the elasticity of the vulcanization products. However, aromatic process oils have the disadvantageous feature of leaving more or less extensive brown to black, sometimes oily, stains on the surface after extended contact with the surfaces of other objects, such as, e.g., floors. Such stains can be discerned, e.g., at tire storage facilities, conveyor belts, driving belts, seals, tubes, shoe soles, and other rubber products. Such stains can hardly be removed.
Additionally, vulcanization products with aromatic process oils generally contain carbon black as a filler, and have the disadvantage that they leave black marks on objects by friction with the surfaces, such as those that are clearly discernible on the landing strips of airports caused by the wear of airplane tires.
In order to produce non-staining vulcanization products whose physical characteristics are not influenced by softeners used, DE 39 41 246 A1 and its family member GB 2,239,870, which are incorporated by reference herein in their entireties, suggest using softeners for rubber compositions containing carbon black that are selected from esters (e.g., dioctylphthalate), naphthenic oils, paraffinic oils, or their mixtures, and furthermore adding a liquid polymer (e.g., liquid polyisoprene) and/or asphaltenes. However, these vulcanisates still have the disadvantage of leaving black markings on the surfaces due to friction. Additionally, problems in processing arise in such rubber compositions. The specific components of the compound cannot easily be homogeneously mixed, and oily perspiration is discernible on the vulcanized products.
EP 0 708 137 A1, which is incorporated by reference herein in its entirety, describes vulcanized products said to lack the disadvantage of leaving black markings by friction and dark discolorations of surfaces due to an extended contact of the vulcanized products with such surfaces. For this purpose, it is suggested for the rubber composition not to contain any aromatic process oils, but to be provided with at least one softener (e.g., dioctylphthalate), liquid polymer, and/or asphaltenes, and, as fillers, silica and carbon black in the volume ratio of 1:1 to 20:1. This rubber composition has processing problems as well in that a completely homogenous mixing is hardly achievable, and a perspiration is also discernible at the vulcanization here as well. Additionally, ecological and/or health concerns are suspected for phthalate softeners. Furthermore, the liquid polymers suggested in EP 0 708 137 A1 are presently expensive and hardly available in sufficient amounts. At present, few types of polymers are available in a liquid form, e.g., polyisoprene.
The present invention provides a sulfur vulcanizable rubber composition that is easily processed, and mixed, and which does not have the disadvantage of leaving black markings due to friction and dark discolorations on surfaces due to an extended contact of the vulcanisate with such surfaces. Additionally, the vulcanization products (rubber products), e.g., tires with a tread made from this compound, do not have disadvantageous effects on their physical characteristics, especially when compared to the products made from known rubber compositions.
The objects according to the present invention can be attained in a sulfur vulcanizable rubber composition which does not contain aromatic process oils that has, as a softener 5 to 60 phr of at least one mineral oil softener having a content of polycyclic aromatic compositions of less than 3 wt-%, determined with the DMSO (Dimethylsulfoxide) extract according to the IP 346 method, with respect to the total weight of the mineral oil softener, and a glass transition temperature below xe2x88x9245xc2x0 C., and contains 1 to 20 phr of at least one of at least one glyceride and/or factice, and has a weight ratio of finely dispersed, precipitated silica to carbon black of 1:2 to 20:1.
Thus, the present invention is directed to a sulfur vulcanizable rubber composition which does not contain aromatic process oils, comprising at least one diene elastomer; at least finely dispersed, precipitated silica and carbon black as fillers, said finely dispersed, precipitated silica and said carbon black being present in a weight ratio of silica to carbon black of 1:2 to 20:1; softeners including 5 to 60 phr of at least one mineral oil softener, comprising a content of polycyclic aromatic compositions of less than 3 wt-% in reference to the total weight of the at least one mineral oil softener, determined by DMSO extract according to IP 346 method, and a glass transition temperature less than xe2x88x9245xc2x0 C., and 1-20 phr of at least one of at least one glyceride and at least one factice; and at least one silane coupling agent.
The invention is also directed to a process for producing a sulfur vulcanizable rubber composition which does not contain aromatic process oils, the rubber composition comprising at least one diene elastomer; at least finely dispersed, precipitated silica and carbon black as fillers, the finely dispersed, precipitated silica and the carbon black being present in a weight ratio of silica to carbon black of 1:2 to 20:1; softeners including 5 to 60 phr of at least one mineral oil softener, comprising a content of polycyclic aromatic compositions of less than 3 wt-% in reference to the total weight of the at least one mineral oil softener, determined by DMSO extract according to IP 346 method, and a glass transition temperature less than xe2x88x9245xc2x0 C., and 1-20 phr of at least one of at least one glyceride and at least one factice; and at least one silane coupling agent; the process comprising (a) mixing the at least one diene elastomer, the finely dispersed, precipitated silica, the carbon black, the at least one mineral oil softener, and the at least one of at least one glyceride and at least one factice, and the at least one silane coupling agent, in the absence of a vulcanization system, and, simultaneously, heating the composition to a temperature of up to 180xc2x0 C.; and (b) adding and mixing a vulcanization system at a temperature below the vulcanization temperature. The mixing of (a) can include at least one heating and cooling cycle.
The at least one glyceride can comprise rapeseed oil.
The at least one diene elastomer can be selected from at least one of natural rubber, synthetic polyisoprene, polybutadiene, and styrene-butadiene-copolymers.
The weight ratio of finely dispersed, precipitated silica to carbon black can be 1:1 to 20:1.
The finely dispersed silica can have a nitrogen surface of 35 to 350 m2/g and a pore volume of 0.2 to 3.4 mL/g.
The at least one silane coupling agent can be present in an amount of 0.2 to 30 parts per weight in relation to 100 parts per weight of silica.
The rubber composition can comprise 10 to 80 phr silica and 1 to 60 phr carbon black.
The invention is also directed to a rubber product composed of the rubber composition vulcanized with sulfur. The rubber product can comprise a tread strip of a tire, or a tire including a tread strip, with the tire being at least partially composed of a rubber composition according to the invention vulcanized with sulfur. Moreover, the tread strip can be at least partially composed of the rubber composition.